1. Field of the Invention
The present invention relates to a controller for a battery powered electric vehicle, and more particularly to a controller suitable for an electric car.
2. Description of the Related Art
For a vehicle using a battery (secondary battery) such as an electric car, it is possible for the vehicle to become incapable of running owing to exhausting of charge of the battery during driving.
Therefore, in the past, in order to prevent inoperability of the vehicle, the remaining amount of electric power in its battery is monitored to suppress the amount of electric power consumed when the discharge progresses to a certain amount. A technology is proposed in, for example, Laid-Open No.4-183206 (1992), Japanese Patent Application where a small capacity motor, separate from the main motor, is provided to drive an electric vehicle, and the drive motor is switched from the main motor to the small capacity motor to continue running when the voltage of the battery decreases owing to the progress of discharge.
The technology described above has the following disadvantages due to an increase in the weight of the vehicle, increase in the complexity of the construction, adverse influence owing to over-discharge of the battery, and a change in driving feeling of the vehicle, which are not taken into account.
Firstly, in the conventional technology, the electric vehicle required to mount a large capacity battery by its nature has to additionally mount another motor. Therefore, the conventional technology is opposed to the main design criterion of an electric vehicle that it be as light as possible to lengthen the running distance with a single battery charge, by effectively utilizing the limited capacity of the battery.
Secondly, in order to assure safety in a vehicle of such a type, for example in exiting a railroad crossing or the like, the vehicle must have capability of driving with a large torque for a short time, even if the battery is exhausted in a carotene degree during running on an ordinary road. In the conventional technology, it is difficult to maintain safety because the drive system is switched to a motor driving with a small torque, and the vehicle cannot cope with the situation. Thirdly, in the conventional technology, the switching of motors for driving the vehicle causes a noticeable change in handling characteristics of the vehicle, which can be unpleasant to the driver. The handling of the vehicle is deteriorated, since the running performance decreases in a step-down shape after the switching of the motors.
Fourthly, in the conventional technology, when the voltage of the battery decreases below a certain value, an electric power converting means for controlling electric power cannot output a desired power in some cases. When the vehicle continues to be driven using the small capacity motor in these instances, the battery is over-discharged, which deteriorates its performance and shortens its life.